This invention relates to the extrusion of moldable material and more particularly to an extrusion die for use in such connection.
Conventional dies, whether cylindrical or planar, are provided with a plurality of extrusion holes therethrough. Moldable material to be extruded, is fed in bulk to the compression side of the die in front of rollers or other form of compression means, whereupon such material is compressed into the extrusion passages. If conditions are favorable, the material, after a number of passes of the compression means, emerges as a compacted string of material from each extrusion passage, from which material, pellets are formed by periodically fracturing or otherwise severing the extruded material as it emerges from the die.
Such dies have presented a number of problems. Moldable materials to be pelleted, vary widely in composition. Some are quite viscous, some difficult to consolidate, others compact readily hence die requirements are quite variable. A particular die might work well with one material and not even start with a material apparently differing but slightly from the other.
We found that enlarged openings on the die inlet side contribute to the versatility of the die. The mechanics of this are not fully understood, but are believed to be related to the working of the pad of material compressed between the roller and the die. In the conventional die, the material in the pad must be forced either directly or at least very locally into the die passage. This appears to require a closer match of die to the material than first forcing the material into a relatively larger opening, such as a precompaction chamber, and then extruding the desired pellet diameter.
Another problem encountered in connection with conventional dies, is that of die wear. Moldable material in most cases processes abrasive characteristics, and this results in considerable wear of the die face, particularly along the path or track of the compression means. When the holes do not accept the feed, the wear is aggravated because the material, under the action of the extrusion means, is partly or wholly forced out of and across the far edge of each entry under pressure, and such action produces wear at the edge of each hole along with wear of the die surface over which the extrusion means passes. This, of course, results in the development of heat and probably accounts in large measure for increasing the power/output ratio of the machine.